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Energy & The CellGlycolysis, Cellular
Respiration & Fermentation
Energy• All life requires energy• Therefore cells require energy
– for growth, active transport, synthesis of carbohydrates, lipids, & proteins
• The source of energy for cells is the energy stored in chemical bonds of organic molecules– these molecules = food molecules,
especially carbohydrates (also lipids)– most common is glucose
Recycling Energy
ATP
• Cells store energy in the chemical bonds of sugar, but cannot use it directly
• To use this energy, the cell must transfer the energy in sugar molecules to ATP
• ATP = adenosine triphosphate
Structure of ATP• The base, adenine• The sugar, ribose• Ribose is bound to
a chain of 3 phosphate molecules connected by high energy bonds
Phosphorylation forms ATP
Releasing Energy from ATP
• If the cell needs energy it breaks the last phosphate bond, releasing energy
ATP ADP + P + energy
• Almost all energy requiring processes in cells use ATP as the energy source
Fermentation• So, where does ATP come from?• Fermentation = breakdown of glucose,
yielding ATP, without O2
• The first living organisms were single cells that existed without O2
– Anaerobic– Lack the enzymes needed to break down
energy molecules with O2
• Many types of bacteria and other single celled organisms still use anaerobic processes to convert energy
Types of Fermentation
• 2 kinds:• Alcoholic fermentation:
– occurs in micro-organisms such as yeast
• Lactic acid fermentation: – occurs in bacteria and animal cells
Fermentation in the Cytosol
• Fermentation occurs in the cytosol
• It produces lactic acid or alcohol
• Fermentation begins with the process of glycolysis, which is also part of aerobic respiration.
Glycolysis• Occurs in the cytoplasm of the cell• One molecule of glucose is split into two
molecules of a three carbon compound called pyruvic acid
• 2 molecules of ATP provide the energy to split the glucose molecule
• When glucose splits, it releases enough energy to form 4 molecules of ATP from ADP + P
• Therefore 2 molecules of ATP are gained
Energy of Glycolysis
The Role of NAD in Glycolysis
• During the conversion of glucose to pyruvic acid, hydrogen is released
• This hydrogen is picked up by a coenzyme, nicotinamide adenine dinucleotide (NAD)
• NAD is a hydrogen acceptor• When it accepts hydrogen, becomes
NADH2
Summary of Glycolysis - Investment
Summary of Glycolysis - Payoff
Energy of Fermentation
• As a result of fermentation, each molecule of glucose yields 2 molecules of ATP
• These ATP molecules come from glycolysis– Fermentation produces no ATP beyond
glycolysis• It removes pyruvic acid, and recycles
NAD, which allows glycolysis to continue, producing ATP
Alcoholic Fermentation• Pyruvic acid from glycolysis combines
with H from NADH2 to produce ethyl alcohol
• 2CH3COCOOH + 2NADH2 2CH3CH2OH + 2CO2 + 2NAD
– CO2 is a waste product
Alcoholic Fermentation Pathway
Lactic Acid Fermentation
• Pyruvic acid combines with H from NADH2 to produce lactic acid:
2CH3COCOOH + 2NADH2 2CH3CHOHCOOH + 2NAD
• Unlike alcoholic fermentation, no CO2 is given off
• Occurs in human cells when there is not enough O2 available for aerobic respiration
Lactic Acid Fermentation Pathway
Uses of Lactic Acid Fermentation
• During strenuous exercise glycolysis occurs at a high rate
• Pyruvic acid is produced rapidly• Muscle cells may not receive enough O2 to
process pyruvic acid through aerobic respiration
• Therefore muscles produce lactic acid which permits glycolysis to continue to supply ATP to your muscles
• When lactic acid builds up, your muscles ache• O2 you take in from heavy breathing helps
convert lactic acid back to pyruvic acid
Cellular Respiration
• Most cells produce ATP by breaking the energy containing bonds of glucose in the presence of oxygen
• Production of ATP this way = Respiration
• Uses O2 to break sugars down to CO2 & H2O– Not the same as breathing
– provides O2, but otherwise quite different
• This process occurs in the many mitochondria of each cell
The Process of Cellular Respiration
• C6H12O6 + 6 O2 6 CO2 + 6 H2O + energy
(sugar) (ATP)
• Two stages of Cellular Respiration:– Anaerobic
• without oxygen
– Aerobic • with oxygen
Cellular Respiration Overview
Anaerobic Stage• The anaerobic stage of cellular respiration is
glycolysis, the same pathway used in fermentation
• This part of cellular respiration occurs in the cytoplasm
• Recall the energy budget for glycolysis:– One molecule of glucose is split into two
molecules of a three carbon compound called pyruvic acid
– 2 molecules of ATP provide the energy to split the glucose molecule
– When glucose splits, it releases enough energy to form 4 molecules of ATP from ADP + P
– Therefore 2 molecules of ATP are gained
Energy of Glycolysis
Aerobic Stage
• After glycolysis, the chemical bonds of pyruvic acid are broken down in a series of chemical reactions
• These occur in the mitochondria and require O2
• The aerobic stage has two parts:– The Citric Acid Cycle– The Electron Transport Chain
Pyruvate Forms Acetyl CoA
The Citric Acid Cycle• Steps to break down pyruvic acid:• In the presence of O2, pyruvic acid breaks
down to acetic acid and CO2 – CO2 is released as waste
• Acetic acid combines with coenzyme A acetyl CoA– This step also forms NADH2 from NAD
• Acetyl CoA enters the citric acid cycle and combines with a 4 carbon compound to produce citric acid
• As the cycle continues, citric acid is broken down in a series of steps, back to the original 4 carbon compound
Energy from the Citric Acid Cycle
• For each molecule of acetyl CoA that enters the cycle, 8 atoms of H are released.
• These hydrogen atoms are trapped by NAD, forming NADH2.
• Therefore, each turn of the cycle yields 4 NADH2
The Electron Transport Chain
• NADH2 releases the hydrogen atoms trapped during glycolysis & the citric acid cycle– Therefore NADH2 becomes NAD again
• Electrons contained in the H atoms pass through a series of coenzymes which are electron acceptors.
• Each time an electron moves from one acceptor to another, an electron is released
• The electron released is used to form molecules of ATP from ADP + P
• This whole process = electron transport chain
Oxygen & The Electron Transport Chain
• The last part of the chain is the electron acceptor, oxygen
• Electrons combine with oxygen & hydrogen to form H2O, which is released as a byproduct
Chemiosmosis
• The process of formation of ATP during the ETS of aerobic respiration as the result of a pH gradient across the membrane of the cristae in the mitochondria = chemiosmosis
• Steps:– H+ ions from the matrix are pumped into the space
between the cristae and the outer membrane.– A H+gradient develops between the inside and
outside of the cristae– This pH differential creates free energy– H+ pass back across the membrane through F1
– O2 is the final H+/ electron acceptor producing H2O
Picturing Chemiosmosis
Cellular Respiration Summary• Thus for every molecule of glucose that is
broken down by glycolysis and respiration, 38 molecules of ATP are formed– Used 2 ATP to begin the process Therefore 36 ATP
gained
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